Remodulating channel selectors for wdm optical communication systems

ABSTRACT

The present invention provides a remodulating channel selector for a wavelength division multiplexed optical communication system. The remodulating selector receives a WDM input signal, selects a particular optical channel from the WDM signal and places the information from the selected signal onto a newly-generated optical output signal. The wavelength of the output optical signal can be the same as or different from one of the optical channels which comprises the WDM input signal. When used in a WDM optical communication system with remodulators at the transmission input, the remodulating selectors provide complete control over the interfaces with optical transmitters and receivers, permitting use with a broad range of optical equipment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/618,684 filed Jul. 17, 2003, now U.S. Pat. No. 7,369,780, issued Apr.16, 2008, which is a continuation of U.S. patent application Ser. No.09/793,645 filed Feb. 26, 2001, now U.S. Pat. No. 6,618,176, issued Sep.9, 2003, which is a continuation of U.S. patent application Ser. No.09/428,420 filed Oct. 28, 1999, now U.S. Pat. No. 6,233,077, issued May15, 2001, which is a divisional of U.S. patent application Ser. No.08/955,058, filed Oct. 21, 1997, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 08/669,049filed Jun. 24, 1996, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/624,269, filed Mar. 29, 1996, which is acontinuation-in-part of U.S. patent application Ser. No. 08/438,844,filed May 11, 1995, now U.S. Pat. No. 5,504,609, issued Apr. 2, 1996,the disclosures of which are incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to wavelength division multiplexed opticalcommunications generally and, more particularly, to remodulating channelselectors for selecting a particular channel wavelength and placing theinformation from that channel wavelength onto a newly generated opticalchannel.

BACKGROUND OF THE INVENTION

Optical communication systems are a substantial and fast-growingconstituent of communication networks. The expression “opticalcommunication system,” as used herein, relates to any system which usesoptical signals to convey information across an optical waveguidingmedium. Such optical systems include, but are not limited to,telecommunications systems, cable television systems, and local areanetworks (LANs). Optical systems are described in Gowar, Ed. OpticalCommunication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure ofwhich is incorporated herein by reference. Currently, the majority ofoptical communication systems are configured to carry an optical channelof a single wavelength over one or more optical waveguides. To conveyinformation from plural sources, time-division multiplexing isfrequently employed (TDM). In time-division multiplexing, a particulartime slot is assigned to each signal source, the complete signal beingconstructed from the portions of the signals collected from each timeslot. While this is a useful technique for carrying plural informationsources on a single channel, its capacity is limited by fiber dispersionand the need to generate high peak power pulses.

While the need for communication services increases, the currentcapacity of existing waveguiding media is limited. Although capacity maybe expanded e.g., by laying more fiber optic cables, the cost of suchexpansion is prohibitive. Consequently, there exists a need for acost-effective way to increase the capacity of existing optical waveguides.

Wavelength division multiplexing (WDM) has been explored as an approachfor increasing the capacity of existing fiber optic networks. In a WDMsystem, plural optical signal channels are carried over a singlewaveguide, each channel being assigned a particular wavelength. Throughthe use of optical amplifiers, such as doped fiber amplifiers, pluraloptical channels are directly amplified simultaneously, facilitating theuse of WDM systems in long-distance optical networks.

To provide compatibility of WDM systems with existing networks, it isdesirable to convert a signal from a received transmission wavelengthfrom a customer to a specific channel wavelength within the WDM system.This is particularly true in WDM systems employing many channels, oftenreferred to as “dense” WDM, where channel spacings are on the order ofone nanometer or less. Such WDM systems require precise control of theoptical signal wavelength for each channel in order to avoid“crosstalk,” i.e., interference between adjacent channels. A WDM opticalsystem for converting signals from received transmission wavelengths tospecific channel wavelengths using optical remodulators is described inU.S. Pat. No. 5,504,609. A WDM optical system which uses bothremodulators and diverse optical Sources (e.g., to accommodate signalswhich are generated at the proper channel wavelength or optical channelsbeing routed from another optical path) is described in parentapplication Ser. No. 08/624,269, the disclosure of which is incorporatedby reference, above.

While both the described approaches advantageously offer compatibilitywith existing optical communication systems, particularly those systemsusing SONET terminal receivers which conform to the SONET “long-haul”standard, i.e., terminals configured to detect low optical signallevels, it would be desirable to provide an output channel signalconforming to SONET “short-reach” interface standards, i.e., terminalsconfigured to detect higher-level optical signals.

Previously, attention has been focused on conversion of a singletransmission channel from a wavelength outside the wavelength bandamplified by optical amplifiers to a wavelength within the wavelengthband amplified by optical amplifiers and then back to the originaltransmission wavelength for reception by an optical receiver. U.S. Pat.No. 5,267,073 describes wavelength conversion in a conventional singlechannel optical system to enable signal amplification by opticalamplifiers. In the patent, an adapter is provided to receive atransmission optical signal having a wavelength which is outside theoperating parameters of the optical amplifier. The signal is supplied toan optical-to-electronic converter module. The resultant electricalsignal is output to an adjusting means comprising a laser pilotingcircuit for directly modulating a signal laser. The output of the signallaser is subsequently amplified by an optical amplifier. At the receiveend, an adapter is provided to convert the optical signals from theamplifier into electrical signals which are fed to an adjustment module.The adjustment module comprises a laser piloting circuit which controlsa laser transmitter. In this manner, the patent purports to avoidproblems in which normal optical line receivers have problems withfrequency response when they are coupled to optical amplifiers inoptical fiber lines.

There is a need in the art for WDM channel selectors which can bothselect a particular channel wavelength from a multiplexed optical signaland place the information from that channel onto a newly-generatedoptical signal, the optical signal selected to have the desiredcharacteristics which make it compatible with the selected terminalreceiver equipment. Such channel selectors would permit the use of lessexpensive terminal equipment, facilitating the use of wavelengthdivision multiplexing in a greater variety of telecommunicationsapplications.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a remodulating channel selector for awavelength division multiplexed optical communication system. Theremodulating selector receives a WDM input signal, selects a particularoptical channel from the WDM signal, and places the information from theselected signal onto a newly-generated optical output signal. Theremodulating selector typically comprises an optical input portconfigured to receive a wavelength division multiplexed opticalcommunication signal from a wavelength division multiplexed opticalcommunication system. An optical channel selector optically communicateswith the optical input port to select a single optical channel from theWDM input signal. An optical-to-electrical converter opticallycommunicates with the optical channel selector to receive the selectedchannel and output an electrical signal which corresponds to informationfrom the selected optical channel. This information is placed onto anoptical signal created by an optical signal emitter such as a laser orlight-emitting diode (LED). When used in a WDM optical communicationsystem with remodulators at the transmission input, the remodulatingselectors provide complete control over the interfaces with opticaltransmitters and receivers, enabling a WDM system to interface with abroad range of optical transmitting and receiving equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an optical communication system employingremodulating channel selectors according to the present invention.

FIG. 2 schematically depicts a remodulator used in the opticalcommunication system of FIG. 1.

FIG. 3 illustrates a block diagram of a forward error correction encoderin accordance with the present invention.

FIG. 4 schematically depicts a remodulating channel selector used in theoptical communication system of FIG. 1.

FIG. 5 illustrates a block diagram of a forward error correction decoderin accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings in detail in which like reference numeralsindicate the same or similar elements in each of the several views, FIG.1 depicts an optical communication system 10 according to an embodimentof the present invention. Optical communication system 10 operates totake optical transmission signals from diverse optical transmitters andother optical signal sources and map the signal sources onto awavelength division optical communication system, i.e., a communicationsystem in which individual optical signals correspond to opticalchannels within a wavelength division multiplexed optical signal carriedon an optical waveguide. Optical communication system 10 includes afirst set of one or more optical transmitters 22, 24, each opticaltransmitter emitting an information-bearing optical signal at a firsttransmission wavelength.

Optical transmitters 22 and 24 generally include a laser, such as a DFBsemiconductor laser, and a modulator for creation of aninformation-bearing optical transmission signal. In an exemplaryembodiment, optical transmitters 22 and 24 are SONET OC-48 transmitters.When plural optical transmitters are provided, the transmissionwavelength of each transmitter can be the same or different. Becausetransmitters 22 and 24 may form part of an existing optical system, awide variety of transmitters emitting in a broad range of wavelengthscan be accommodated in the optical communication system of the presentinvention, thus ensuring compatibility with currently-deployedtransmission equipment. Typical transmission elements emit wavelengthsranging from about 1300 to 1600 nm. Transmission elements in currentoptical communication systems and various optical modulation techniquesemployed therein are described in Gowar, Optical Communication Systems,incorporated by reference above. In the depicted exemplary embodiment,optical transmitter 22 is a 1310 nm optical transmitter and opticaltransmitter 24 is a 1550 optical transmitter, commercially availablefrom NEC, Fujitsu, Alcatel, and Nortel.

Optical communication system 10 includes a plurality of remodulators 30for receiving the transmitted information-bearing optical signal attransmission wavelength from the first set of optical transmitters andtransmitting an information-bearing optical signal at a WDM opticalsystem channel wavelength. The expression “information-bearing opticalsignal,” as used herein, refers to an optical signal which has beencoded with information, including, but not limited to, audio signals,video signals, and computer data, generally through modulation.Similarly, the expression “non-information-bearing optical signal,” asused herein, relates to a CW optical signal which has not been codedwith information, e.g., an optical carrier which has not been modulated.Typically, the wavelengths emitted by the remodulators are selected tobe within the 1500 nanometer range, the range in which the minimumsignal attenuation occurs for silica-based fibers. More particularly,the wavelengths emitted by the remodulators are selected to be in therange from 1530 to 1560 nanometers. However, other wavelength bands maybe selected according to overall system requirements.

For a 16-channel wavelength division multiplexed optical communicationsystem, an exemplary channel plan is set forth in Table 1 below. Theoptical channel plan dictates both the wavelengths of the opticalremodulators and the corresponding wavelengths selected by thewavelength selectors in the optical receiving systems.

TABLE 1 Channel Number Wavelength (nm) 1 1557.36 2 1556.56 3 1555.75 41554.94 5 1554.13 6 1553.33 7 1552.52 8 1551.72 9 1550.92 10 1550.12 111549.32 12 1548.51 13 1547.72 14 1546.92 15 1546.12 16 1545.32

Wavelength division multiplexed topical communication system 10optionally includes a second set of one or more optical transmitters 25which directly output an optical signal having a wavelength whichcorresponds to an optical channel within the channel plan of thecommunication system. Consequently, optical transmitters 25 opticallycommunicate with optical multiplexer or combiner 50 without the need forremodulation by remodulators 30. Optical transmitters 25 arecommercially available from a variety of suppliers, including theOCT-204 series of optical transmitters from Alcatel, the HT2H-LR1Hoptical transmitters from Fujitsu, and the ITS-2400 optical transmittersfrom NEC.

Wavelength division multiplexed optical communication system 10 furtheroptionally comprises additional sources of optical signals, e.g.,optical signals from add/drop multiplexers or demultiplexers from otheroptical systems. Examples of various configurations of WDM opticalsystems with diverse signal sources are described in the parentapplication, incorporated by reference above.

An exemplary remodulator 30 for use in optical communication system 10is schematically depicted in FIG. 2. In remodulator 30, the transmittedoptical signal is converted by electro-optical converter 31, typically aphotodiode, to an electrical signal. The electrical signal is amplifiedby transimpedance amplifier 32, passed through filter 33 to limit thenoise bandwidth and waveshape the signal, and further amplified bylimiting amplifier 34. Optionally, remodulator 30 can include clock anddata recovery circuit 40 for use with high data rate signals. Switch 41automatically selects high data rate signals and passes them throughclock/data recovery element 43. The selected signals are retimed,advantageously reducing jitter. The retimed signal exits clock and datarecovery circuit through switch 42.

The resultant electrical signal is used to drive external modulator 36through modulator driver 37. As used herein, the expression “externalmodulator” includes any modulator which acts on an optical carrieremitted from a continuous wave (CW) optical source, such as a laser.Such external modulators can be packaged with the optical source orpackaged separately from the optical source.

Remodulator 30 also includes a optical source, such as laser 37, fordelivering a non-information-bearing optical carrier signal to laseroutput waveguide 39. In an exemplary embodiment, laser 37 is a DFBsemiconductor diode laser, generally comprising one or more III-Vsemiconductor materials, commercially available from a wide variety ofsuppliers such as Fujitsu, GEC Marconi, Alcatel, and Hewlett-Packard.The laser outputs an optical carrier signal at a particular channelwavelength, the wavelength corresponding to a channel selectorwavelength included in the remodulating channel selector. Laser control38 provides the required laser bias current as well as thermal controlof the laser. Using thermal control, the precise operating wavelength ofthe laser is maintained, typically to within a one angstrom bandwidth.

External modulator 36 acts on the optical carrier signal output fromlaser 37, as opposed to acting on the laser itself or on a laser driver,as occurs in direct modulation systems. An exemplary external modulatoremploys a waveguiding medium whose refractive index changes according tothe applied electrical field, i.e., a material exhibiting anelectro-optic effect. Consequently, the phase of input optical carriersignals is altered as a result of the changing refractive index of theoptical waveguide. A suitable electro-optic waveguiding material for theexternal modulators of the present invention is lithium niobate, LiNbO₃.An exemplary electro-optic modulator for use as external modulator 36 isa Mach-Zehnder interferometric modulator which provides high-speedintensity modulation of optical carriers. In the Mach-Zehnderconfiguration, two optical paths are provided. An incoming opticalcarrier is split between the two paths of the interferometer. At leastone path of the interferometer is phase modulated. When the signal isrecombined at the output, the light from the paths either constructivelyor destructively interferes, depending upon the electrical field appliedto the surrounding electrodes during the travel time of the carrier,creating an amplitude-modulated output signal. Further details ofelectro-optic modulators are described in Becker, “Broad-Band GuidedWave Electrooptic Modulators,” IEEE Journal of Quantum Electronics, Vol.QE-20, No. 7, July, 1984, pp. 723-727, the disclosure of which isincorporated by reference herein. Mach-Zehnder interferometers suitablefor use in external electro-optic modulator 36 are commerciallyavailable from United Technologies, and IOC. The modulated output signalis the information-bearing optical channel whose wavelength correspondsto a particular channel selector wavelength in the optical communicationsystem.

Alternatively, the external modulator employed in the remodulators ofthe present invention can be selected from electro-absorption externalmodulators. Electro-absorption modulators function by changing thebandgap of the modulating material to impart information to the opticalcarrier signal. Exemplary electro-absorption modulators are described inWood, “Multiple Quantum Well (MQW) Waveguide Modulators,” Journal ofLightwave Technology, Vol. 6, No. 6, June, 1988, pp. 743-757, thedisclosure of which is incorporated by reference herein. Optionally, theremodulator can include means for reduction of non-linear effects, suchas stimulated Brillouin scattering (SBS), in the optical communicationsystem. Suitable device and techniques for reduction of non-lineareffects which can be employed in the optical communication system of thepresent invention are described in U.S. Pat. Nos. 6,116,821, 5,200,964,5,257,124, and 5,295,209, incorporated by reference herein.

Optionally, remodulators 30 include forward error correction (FEC)encoders 45. The addition of forward error correction to a WDM opticalcommunication system advantageously decreases the bit error rate (BER)by adding redundancy, e.g., coding bits, to the individual opticalchannels which comprise the WDM signal. In particular, the addition ofFEC permits the WDM system to achieve substantially error-freeperformance in the presence of the nonlinearities present in opticalcommunication system. At the receive end, a forward error correctiondecoder examines the coding bits to accurately reconstruct thetransmitted information. A variety of coding algorithms may be used toaccomplish forward error correction in the WDM optical systems of thepresent invention. Exemplary algorithms are convolutional encoding withthreshold decoding, Viterbi decoding, or Reed-Solomon encoding/decoding.Detailed descriptions of these and other coding algorithms are found inWiggert, Error-Control Coding and Applications, (Artech House, c. 1978),the disclosure of which is incorporated by reference herein.

Advantageously, forward error correction in the WDM optical systems ofthe present invention enables a “channel trace” function that encodesthe channel ID, source, and destination into a small overhead bit streamwhich would permit the remodulating channel selector to respond only toan incoming signal with the proper addressing. The use of channeltracing through forward error correction additionally permits channelpath trace through the WDM system, a particularly useful feature forcomplex system topologies and WDM systems with extensive add/dropmultiplexing or cross-connect features.

An exemplary encoder 45 is shown in greater detail in FIG. 3. Signalsreceived from clock/data recovery circuit 43 are supplied to encoder 45on input line 410. Serial-to-parallel converter circuit 412 converts thereceived serial data to parallel data. The output of serial-to-parallelconverter circuit 412 is supplied on a plurality of lines 413 to FECencoder core circuit 414, as described, for example, in U.S. Pat. No.6,308,295 issued Oct. 23, 2001, and entitled “Parallel SpectralReed-Solomon Encoder and Decoder” to Neifeld et al., incorporated byreference herein. FEC encoder core circuit 414 encodes the received datain parallel in accordance with a Reed-Solomon code by attaching aplurality of syndrome symbols or groupings of bits followed by aninverse Fourier transform of the data and syndromes. FEC encoder corecircuit 414 outputs encoded data in parallel to parallel-to-serialconverter 416, which serializes the data for output to modulator drive35.

Returning to FIG. 1, each information-bearing optical signal produced bya remodulator constitutes a channel in optical system 10, the wavelengthof which corresponds to a channel selector wavelength. The opticalsignal channels output from remodulators 30 are brought together inoptical combiner 50 for conveyance to optical waveguide 60. Opticalcombiner 50 is selected from any passive optical component which cancombine plural wavelengths into a single output medium. Frequently,optical splitters used to divide a signal among plural outputs are usedas optical combiners, operated in reverse fashion from the splitter.Exemplary optical combiners include 1.times.N passive splittersavailable from Coming, Inc., Corning, N.Y., 1.times.N wideband singlemode splitters available from IOT Integrierte Optik GmbH,Waghausel-KiiTlach, Germany, and fused fiber combiners available fromGould, Inc., Millersville, Md. The combination of channels forms amultiplexed optical signal which is output to waveguide 60. Opticalwaveguide 60 is typically a single-mode optical fiber such as SMF-28,available from Coming, and TRUEWAVE, available from AT&T Corp./LucentTechnologies, and is the principal transmission medium for the opticalcommunication system. However, any optical waveguide which is capable oftransporting multiple optical wavelengths can be employed as waveguide60 in optical system 10.

Interposed along optical waveguide 60 are one or more optical amplifiers70. Optical amplifiers 70 are selected from any device which directlyincreases the strength of plural optical signals without the need foroptical-to-electrical conversion. In general, optical amplifiers 70 areselected from optical waveguides doped with rare earth ions such aserbium, neodymium, praseodymium, ytterbium, or mixtures thereof. Opticalamplifiers, their materials, and their operation are further describedin Gowar, Ed. Optical Communication Systems, incorporated by referenceabove and in Desurvire, Erbium-Doped Fiber Amplifiers, (John Wiley &Sons, Inc., N.Y.), c. 1994, the disclosures of which are incorporated byreference herein. Exemplary optical amplifier configurations aredescribed in the parent applications, the disclosures of which areincorporated by reference. Further descriptions of doped-fiber opticalamplifier configurations suitable for use in the optical communicationsystem of the present invention are described in Bjarklev, Optical FiberAmplifiers: Design and System Applications, (Artech House, Norwood,Mass.) c. 1993, the disclosure of which is incorporated herein byreference.

Following transmission and amplification of the multiplexed opticalsignals along waveguide 60, a portion of the multiplexed optical signalmust be sent to each of the remodulating channel selectors for selectionand routing to an appropriate optical receiver. The multiplexed signalis input to optical splitter 90 which places a portion of themultiplexed signal onto plural output paths 92. Each output path 92optically communicates with a remodulation channel selector 100. Opticalsplitter 90 is selected from any optical device which can divide aninput optical signal and place it onto plural output paths. Exemplarysplitters include passive optical components such as those componentsdescribed for use as optical combiner 50. Splitter 90 in combinationwith remodulating channel selectors 100 constitute an exemplarywavelength demultiplexer.

FIG. 4 schematically depicts an exemplary remodulating channel selector100 for use in WDM optical communication system 10. Remodulating channelselector 100 includes an optical input port 101 for receiving the WDMoptical signal from splitter output path 92. The WDM optical signaltraverses optical path 105 through splitter 103 to channel selector 102.Channel selector 102 passes optical signals having wavelengths otherthan the channel wavelength to be processed by the remodulating channelselector. These non-selected channels pass through low reflectivity port104 and exit the optical communication system. The low reflectivity port104 may be an angled fiber cut, although any low reflectivity waveguidetermination technique may be employed. The selected channel wavelengthis reflected by channel selector 102 through splitter 103 onto opticalpath 106. In an exemplary embodiment, optical splitter 103 is a fusedfiber coupler and channel selector 102 comprises a Bragg grating memberconfigured to reflect the selected channel wavelength. Preferably, theBragg grating comprises a series of photoinduced refractive indexperturbations in an optical fiber which causes the reflection of opticalsignals within a selected wavelength band. Bragg gratings suitable foruse in the optical system of the present invention are described inMorey et al., “Photoinduced Bragg Gratings in Optical Fibers,” Opticsand Photonics News, February 1994, pp. 8-14, the disclosure of which isincorporated by reference herein.

Although a Bragg grating is depicted as the channel selecting element,it is understood that numerous other optical components can be employedas channel selector 102. Such optical components include, but are notlimited to, multilayer interference filters, tunable Fabry-Perotselectors, and wavelength routers. In an exemplary embodiment, theoptical bandwidth is selected to be sufficiently narrow to minimize thedeleterious effects of amplified spontaneous emission (ASE).

The selected optical channel is converted by electro-optical converter108, typically a photodiode, to an electrical signal. The electricalsignal is amplified by transimpedance amplifier 110 and routed throughclock and data recovery circuit 112 for retiming. In an exemplaryembodiment, the electrical bandwidth of the optical-to-electricalconverter and the transimpedance amplifier is selected to match the datarate of the incoming signal. Optionally, the remodulating channelselector includes FEC decoder 114 circuit for accurate reconstruction ofthe transmitted signal, as discussed above.

As shown in FIG. 5, FEC decoder 114 includes a serial-to-parallelconverter 510, and FEC decoder core circuit 512 and a parallel-to-serialconverter circuit 514. Data from clock/data recovery circuit 112 issupplied to a serial-to-parallel converter 510, which supplies aparallel output to FEC decoder core circuit 512, as described, forexample, in Neifeld et al., supra. As further described in Neifeld etal., FEC decoder core circuit 512 includes a Fourier transform circuit,Berlekamp algorithm circuit and Recursive Extension circuit (not shown).Received data is decoded by initially performing the Fourier transform.The data is next typically supplied to both a temporary memory and theBerlekamp algorithm circuit, which acts on the data in parallel tosupply a parallel output to the Recursive Extension circuit. Therecursive extension circuit, also operates in parallel, to generate anerror signal, which is compared with the received data stored in memory.As a result, errors which may have occurred during transmission, forexample, are corrected. The resulting parallel output of FEC decodercore circuit 512 is supplied to parallel-to-serial conversion circuit514 and passed to modulator 118. The parallel construction of FECdecoder 114, as well as FEC encoder 45 described above, permits encodingand decoding of data at high speeds.

Returning to FIG. 4, direct modulation of optical transmitter 116 by wayof modulator 118 will now be described. Although “modulator” 118 isdepicted as a discrete device, in actuality it can consist of d.c. powersupply 119 interconnected with an electronic switch. The electronicswitch in turn optically communicates with the electrical signalcontaining the information from the selected channel output through theoptical-to-electrical converter and processed by the subsequentelements. The electronic switch controls the flow of current from thed.c. supply to the optical emitter in response to the informationreceived from the electrical signal. Alternatively, the directmodulation of the emitter can be performed using a voltage-controlledcurrent source for the elements labeled 118 and 119 in FIG. 3. Such acurrent source for the optical emitter provides a current whosemagnitude is directly related to the applied voltage. The appliedvoltage represents the information received from the optical-toelectrical converter; alternatively the current may be directly derivedfrom the modulating voltage.

Optical transmitter 116 is selected from a variety of optical devices,depending upon the optical interface required for receiver 130. When thesignal emitted by the remodulating channel selector is destined for longdistance transmission (e.g., through the optical combiner of a furtherWDM optical system as depicted in FIG. 1), the optical emitter isselected to be a DFB laser. When the signal emitted by the remodulatingchannel selector is destined for an adjacent receiver, the opticalemitter within the optical transmitter is selected from lower-cost,shorter-range optical emitters such as Fabry-Perot lasers,light-emitting diodes, and superluminescent diodes.

The wavelength of the optical emitter employed in optical transmitter116 can be the same wavelength as the wavelength of the optical channelselected by the particular channel selector or it can be a differentwavelength. When the optical channel is output directly to a receiver,the wavelength of the optical signal is not critical. In such anembodiment, the same wavelength can be output by all of the opticaltransmitters 116. Since an optical signal being output directly to anoptical receiver need not be optically amplified, the optical emittercan be selected to have any wavelength detectable by the opticalreceiver (e.g., a wavelength outside the gain band of rare-earth dopedfiber amplifiers such as 1310 nm). However, if the optical signalemitted by the remodulating channel selector is destined fortransmission in the same or another WDM system, then the wavelength ofthe optical emitter is selected to be compatible with the channel planof that system. For example, the optical emitter may create a signalhaving the same wavelength as the selected optical signal, or it mayproduce an optical signal having a wavelength which corresponds toanother channel from the input WDM optical signal. If the remodulatingchannel selectors are incorporated into a switching matrix, a variablewavelength optical emitter can be used to dynamically select anavailable wavelength within a WDM optical channel plan.

In an alternate embodiment (not shown) the optical signal emitter isexternally modulated, e.g., as in the remodulators of FIG. 2. Externalmodulation is particularly advantageous when the signal output by theremodulating channel selector is destined for long-distancetransmission. In such an embodiment, the remodulating channel selectorscan serve to reshape and retime an optical signal, e.g., an opticalsignal which has traversed the maximum dispersion-limited transmissiondistance (e.g., a distance of approximately 600 km for optical fiberssuch as SMF-28).

In this manner, a modulated optical signal is output by opticaltransmitter 116 through remodulating channel selector output port 113.The remodulating channel selector output signal is transmitted tooptical receiver 130. Receiver 130 generally detects the optical signaland converts it to an electrical signal, typically through the use of aphotodiode device. Various optical receivers suitable for use in opticalsystem 10 are described in Gowar, Optical Communication Systems,discussed above. In optical communication system 10, receiver 130 willfrequently be part of an existing optical communication system to whichthe remodulated optical signal is routed. Consequently, the opticalsystem 10 can function with numerous types of receivers to ensurecompatibility with existing optical equipment. In particular, thepresence of remodulating channel selectors 100 enables the WDM opticalcommunication system to communicate with optical receivers conforming tothe SONET “short-haul” standard. Further descriptions of SONET interfacestandards are found in SONET Transport Systems: Common Criteria,(GR-253-CORE, Issue 1, December, 1994), the disclosure of which isincorporated by reference herein.

Alternatively, as depicted in FIG. 1, the output of a remodulatingchannel selector is conveyed to another WDM optical system or a portionof the same WDM optical system through input to an optical combiner formultiplexing with other optical signals or routing, e.g., through anadd/drop multiplexer.

Optionally, channel selectors 120 are provided for selecting an opticalchannel from an input WDM optical signal. Such channel selectors,described in more detail in the parent applications incorporated byreference above, select an optical channel and directly output theselected channel without remodulation. Such channel selectors are usedparticularly when the optical receivers with which they communicateconform to the SONET “long-haul” standard. Such “non-remodulating”channel selectors can also route their selected optical channels to anoptical combiner for further transmission in the same WDM optical systemor in a different WDM optical system.

While the foregoing invention has been described in terms of theembodiments discussed above, numerous variations are possible.Accordingly, modifications and changes such as those suggested above,but not limited thereto, are considered to be within the scope offollowing claims.

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 20. Anoptical system, comprising: a plurality of optical remodulators, eachbeing configured to receive a respective one of a first plurality ofoptical signals, and output a respective one of a second plurality ofoptical signals in response to a corresponding one of the firstplurality of optical signals, each of the second plurality of opticalsignals being at a respective one of a plurality of wavelengths, whereineach of the plurality of optical remodulators comprise a laser controlto maintain a precise operating wavelength; and an optical combinerconfigured to receive said second plurality of optical signals andsupply said optical signals to an optical communication path.
 21. Theoptical system of claim 20, wherein the laser control comprises thermalcontrol and a laser bias current.
 22. The optical system of claim 20,wherein the laser control is configured to maintain a precise operatingwavelength to within one angstrom bandwidth.
 23. The optical system ofclaim 20, wherein each of the plurality of wavelengths are spectrallyspaced from one another by substantially 0.8 nm.
 24. The optical systemof claim 23, wherein each of the plurality of wavelengths are spectrallywithin a range of 1530 to 1560 nm.
 25. The optical system of claim 20,wherein each of the plurality of optical remodulators are configured toprovide 3R transponder functionality comprising reshaping, reamplifying,and retiming of a received signal.
 26. The optical system of claim 25,wherein each of the plurality of optical remodulators are configured tomonitor SONET overhead on the received signal.
 27. The optical system ofclaim 25, wherein each of the plurality of optical remodulatorscomprise: a clock and data recovery circuit configured to provideretiming of the received signal; an electro-optical converter,amplifier, and filter configured to provide reshaping of the receivedsignal; and a laser and external modulator configured to providereamplifying of the received optical signal.
 28. The optical system ofclaim 20, wherein each of the plurality of optical remodulatorscomprises an external modulator configured to modulate a laser.
 29. Theoptical system of claim 28, wherein the laser control operates on thelaser to maintain a precise operating wavelength of the laser.
 30. Theoptical system of claim 28, wherein the laser control, the laser, andthe external modulator are packaged in a single package.
 31. The opticalsystem of claim 30, wherein one or more of the plurality of remodulatorsand the optical combiner are packaged in a single package.
 32. Theoptical system of claim 20, wherein at least one of the plurality ofremodulators comprises: a forward error correction encoder configured toencode a forward error correction algorithm; and a forward errorcorrection decoder configured to decode the forward error correctionalgorithm.
 33. The optical system of claim 32, wherein the forward errorcorrection algorithm comprises one of convolutional encoding withthreshold decoding, Viterbi decoding, and Reed-Solomonencoding/decoding.
 34. A wavelength division multiplexed system,comprising: an optical combiner configured to receive a plurality ofoptical signals and combine the plurality of optical signals to anoptical communication path; and an optical splitter configured toreceived the combined plurality of optical signals from the opticalcommunication path and to split the combined plurality of opticalsignals into the plurality of optical signals; wherein each of theplurality of optical signals is at one of a plurality of wavelengths;wherein the plurality of wavelengths are spectrally spaced from oneanother by substantially 0.8 nm.
 35. The wavelength division multiplexedsystem of claim 34, further comprising wavelength control configured tomaintain a spectral spacing of each of the plurality of wavelengths. 36.An optical transponder, comprising: an electro-optical converterconfigured to receive a first optical signal and convert the firstoptical signal to a electrical signal; a reshaping mechanism to reshapethe electrical signal; a clock and data recovery circuit to retime theelectrical signal; and an external modulator configured to modulate alaser responsive to the electrical signal, wherein an output of theexternal modulator comprises a second optical signal at one of aplurality of wavelengths, and wherein the second optical signalcomprises a retimed, reshaped, and reamplified version of the firstoptical signal.
 37. The optical transponder of claim 36, furthercomprising: a laser control circuit configured to maintain the one of aplurality of wavelengths spectrally to within at least one angstrom ofbandwidth.
 38. The optical transponder of claim 37, wherein the lasercontrol circuit utilizes thermal control and a laser bias currentcontrol.
 39. The optical transponder of claim 36, wherein a plurality ofoptical transponders are packaged in a single device.